This invention relates to a process for preparing a polyimide shaped article, e.g. a sheet or film, containing pigment particles, such as carbon or alumina, and having improved tensile and dielectric strength, elongation and electrical conductivity.
Pigment containing polyimide articles, such as polyimide film, are well-know in the art and are generally prepared by blending pigment particles into the polyamic acid precursor and then shaping and curing the polyamic acid to form the polyimide.
For example, Canadian Patent 708,896 discloses an electrically conductive polyimide composition prepared by blending conductive carbon particles, e.g. carbon blacks, in the precursor polyamic acid; shaping the particle containing polyamic acid into a structure; and then thermally converting the shaped structure into a polyimide containing the carbon particles.
U.S. Pat. No. 5,078,936, issued to Parish et al on Jan. 7, 1992, discloses an electrically conductive polyimide article prepared by blending carbon black and graphite in a polar solvent to form a slurry, mixing the slurry with a polyamic acid precursor to form a dope, shaping the dope into a structure and then converting the shaped structure into the polyimide article.
Normally, the blending of the pigment particles is performed while the polyamic acid is in solution in an organic solvent, such as N,N-dimethylacetamide. In order to facilitate particle dispersion and maximize wetting of the pigment particles with the polyamic acid polymer, the pigment particles are slurried in the polymer solvent in a ball, sand or Kady mill. Typically, the pigment particles have a higher density than the solvent and tend to settle and reagglomerate. In addition, the level of adhesion between the polyamic acid polymer and the pigment particles is oftentimes very low and results in lowering of the properties of the polyimide film, e.g. decreased dielectric strength, tenacity and elongation.
It has now been found that shaped polyimide articles containing pigment particles can be prepared by first pre-wetting the pigment particles in an organic solvent with a thermoplastic polyamic acid precursor derived from 4,4'-oxydiphthalic dianhydride and an aliphatic and/or aromatic diamine; admixing the precursor treated pigment particles with a polymerization mixture of an aromatic tetracarboxylic acid dianhydride and an aromatic diamine in the organic solvent and thermally or chemically converting the polyamic acid containing the pigment particles to the polyimide. Polyimide articles containing alumina prepared according to the process of the invention have improved tensile strength, elongation and dielectric strength, and polyimide articles containing carbon have improved electrical conductivity. Polyimide articles of this invention are suitable for use in electronics, copier belt, heating tape, opaque film and thermally conductive film applications.